Highly Reduced Genomes of Protist Endosymbionts Show Evolutionary Convergence
- Unrelated bacterial symbionts from marine diplonemids show convergent evolution
- The symbionts have reduced genomes with similar content and metabolic potential
- The symbionts contain secretion systems including the type secretion system
- Diverse symbionts from a large range of eukaryotic hosts have similar modified cellular machinery
Bacterial endosymbionts have evolved multiple times independently across the tree of life. George et al. provide an example of convergent evolution in the endosymbionts of marine protists and reveal the invisible interactions between these bacteria and their hosts.
Genome evolution in bacterial endosymbionts is notoriously extreme: the combined effects of strong genetic drift and unique selective pressures result in highly reduced genomes with distinctive adaptations to hosts [1–4]. These processes are mostly known from animal endosymbionts, where nutritional endosymbioses represent the best-studied systems. However, eukaryotic microbes, or protists, also harbor diverse bacterial endosymbionts, but their genome reduction and functional relationships with their hosts are largely unexplored [5–7]. We sequenced the genomes of four bacterial endosymbionts from three species of diplonemids, poorly studied but abundant and diverse heterotrophic protists [8–12]. The endosymbionts come from two bacterial families, Rickettsiaceae and Holosporaceae, that have invaded two families of diplonemids, and their genomes have converged on an extremely small size (605–632 kilobase pairs [kbp]), similar gene content (e.g., metabolite transporters and secretion systems), and reduced metabolic potential (e.g., loss of energy metabolism). These characteristics are generally found in both families, but the diplonemid endosymbionts have evolved greater extremes in parallel. They possess modified type VI secretion systems that could function in manipulating host metabolism or other intracellular interactions. Finally, modified cellular machinery like the ATP synthase without oxidative phosphorylation, and the reduced flagellar apparatus present in some diplonemid endosymbionts and nutritional animal endosymbionts, indicates that intracellular mechanisms have converged in bacterial endosymbionts with various functions and from different eukaryotic hosts across the tree of life.
George E.E., Husník F., Tashyreva D., Prokopchuk G., Horák A., Kwong W.K., Lukeš J., Keeling P.J. 2020: Highly reduced genomes of protist endosymbionts show evolutionary convergence. Current Biology 30: 925–933. [IF=9.193]